Organic light emitting devices (“OLEDs”), including both polymer and small-molecule OLEDs, are potential candidates for a great variety of virtual- and direct-view type displays, such as lap-top computers, televisions, digital watches, telephones, pagers, cellular telephones, calculators and the like. Unlike inorganic semiconductor light emitting devices, organic light emitting devices are generally simple and are relatively easy and inexpensive to fabricate. Also, OLEDs readily lend themselves to applications requiring a wide variety of colors and to applications that concern large-area devices.
OLEDs generate light when an electron and a hole combine in a light-emitting layer in the OLED to generate a photon. The percentage of combined electrons and holes that result in generation of a photon in the light-emitting layer is referred to as the “internal electroluminescence quantum efficiency.” The percentage of generated photons that are transmitted or “coupled” out of the device is referred to as the “external electroluminescence quantum efficiency” or the “out-coupling” efficiency of the device. Some models predict that only about 20% or less of the generated photons are transmitted out of the device. It is believed that this is at least in part due to the fact that the generated photons are trapped within the OLED device structure by internal reflection at interfaces within the OLED, resulting in waveguiding of the photons within the OLED and absorption of the trapped photons by the OLED. Absorption can occur within any part of the OLED device, such as within an ITO anode layer or within the substrate, where the refractive index of that particular part is greater than the refractive indices of the adjacent parts. Waveguided photons that are not absorbed by the device itself can also be directed out of the edges of the device. These phenomena result in decreased external electroluminescence quantum efficiency and a reduction in the luminescence or brightness in directions extending outwardly from the surface of the device.
Approaches have been advanced to increase the out-coupling efficiency of OLEDs. For example, Yamasaki et al., “Organic light-emitting device with an ordered monolayer of silica microspheres as a scattering medium,” Appl. Phys. Lett., Vol. 76, No. 10 (March 2000), incorporated in its entirety herein by reference, discloses providing an OLED with an ordered monolayer of silica microspheres. Yamaski et al. disclose that the silica microspheres, placed as a hexagonal close-packed array on a top or bottom surface of a glass substrate, behaved as a light scattering medium for light propagated in waveguiding modes within the OLED. Some of the light scattered by the microspheres was transmitted out of the device, increasing the out-coupling efficiency of the OLED.
In another approach, Tsutsui et al., “Doubling Coupling-Out Efficiency in Organic Light-Emitting Devices Using a Thin Silica Aerogel Layer,” Adv. Mater., Vol. 13, No. 15 (August 2001), disclose providing an OLED with a thin silica aerogel layer of refractive index less than 1.03 positioned between an emissive layer and a glass substrate. The external quantum efficiency of an OLED constructed in this manner, as reported by Tsutsui et al., was 1.39% as compared to 0.765% (equating to an “enhancement factor” of about 1.8) for a similar device not provided with the thin silica aerogel layer.
Another factor limiting the practical application of OLEDs is their susceptibility to environmental elements such as moisture and oxygen. Oxygen and moisture can produce deleterious effects on certain OLED structural components, such as reactive metal cathode components. Without protection, the lifetime of the devices can be severely limited. For example, moisture and oxygen are known to increase “dark spot areas” in connection with OLED structures. The organic materials utilized in a conventional OLED structure can also be adversely affected by environmental species such as water and oxygen. Approaches to protecting OLEDs from environmental elements include, as discussed below, providing the OLED with a protective layer or cover that resists permeation by moisture and/or oxygen.
In general, two-dimensional OLED arrays for imaging applications are known in the art and typically include an OLED display area that contains a plurality of active regions or pixels arranged in rows and columns. FIGS. 1A and 1B are simplified schematic representations (cross-sectional views) of OLED structures of the prior art. The OLED structure shown in FIG. 1A includes a single active region 15 which includes an electrode region such as anode region 12, a light emitting region 14 over the anode region 12, and another electrode region such as cathode region 16 over the light emitting region 14. The active region 15 is disposed on a substrate 10. Barrier layer 20 disposed over active region 15 is provided to restrict transmission of oxygen and water vapor from an outer environment to the active pixel 15.
It is known to provide a barrier layer as a multilayer structure comprising an alternating series of one or more polymeric “planarizing” sublayers and one or more “high density” sublayers of inorganic or dielectric material.
The polymer multilayer (“PML”) process is advantageous because it is a vacuum compatible process which produces a conformal coating that does not require the separate attachment of a preformed cover for protecting an OLED from environmental elements. Moreover, the PML process produces a composite barrier layer with good resistance to moisture and oxygen penetration. A PML composite barrier layer can also be disposed on any surface of a substrate, such as between a top surface of the substrate and the active region. The use of a PML composite barrier layer disposed on a substrate is particularly advantageous when the substrate is permeable to oxygen and moisture, as is often the case with polymeric substrates used in constructing flexible OLEDs (FOLEDs). As the name suggests, these structures are flexible in nature. Examples of OLEDs protected with PML composite barrier layers are disclosed in, for example, U.S. Pat. Nos. 5,757,126, 6,146,225 and 6,268,295 all of which are incorporated herein in their entireties.
In one common OLED structure such as shown in FIG. 1A, light from the light emitting layer 14 is transmitted downwardly through the substrate 10. In such a “bottom-emitting” configuration, the substrate 10 and anode 12 are formed of transparent materials. The cathode 16 and barrier layer 20 need not be transparent in this configuration. Moreover, structures are also known in which the positions of the anode 12 and cathode 16 in FIG. 1A are switched as illustrated in FIG. 1B. Such devices are sometimes referred to as “inverted OLEDs”. In such an inverted OLED bottom-emitting configuration as illustrated in FIG. 1B, the cathode 16 and substrate 10 are formed of transparent materials.
However, other OLED architectures are also known in the art, including “top-emitting” OLEDs and transparent OLEDs (or “TOLEDs”). For top-emitting OLEDs, light from the light emitting layer 14 is transmitted upwardly through barrier layer 20. In a top-emitting configuration based on a design like that shown in FIG. 1A, the cathode 16 and barrier layer 20 are formed of transparent materials while the substrate 10 and anode 12 need not be transparent. In an inverted top-emitting OLED configuration based on a design like that shown in FIG. 1B, the anode 12 and barrier layer 20 are formed of transparent materials. In this configuration, the cathode 16 and substrate 10 need not be transparent.
For TOLEDs, in which light is emitted from both the top and bottom of the device, the substrate 10, anode 12, cathode 16 and barrier layer 20 are formed of transparent materials. TOLEDs can be based on a configuration such as that shown in either FIG. 1A or FIG. 1B.